1. Field of the Invention
The present invention relates to Venetian blinds and, more specifically, to a screw transmission mechanism for a motor-driven blind.
2. Description of the Related Art
A regular Venetian blind comprises headrail, a bottom rail, a plurality of slats arranged in parallel between the headrail and the bottom rail, an amplitude modulation control mechanism for controlling lifting and positioning of the bottom rail to change the extending area of the blind, a frequency modulation control mechanism for controlling the tilting angle of the slats to regulate the light. The amplitude modulation control mechanism comprises an endless lift cord suspended from the headrail at one lateral side for pulling by hand to lift/lower the bottom rail. The frequency modulation control mechanism comprises a frequency modulation member disposed at one lateral side of the blind for permitting rotation by the user to regulate the tilting angle of the slats. When adjusting the elevation of the bottom rail, the user must approach the blind and pull the lift cord by hand with much effort. Further, because the lift cord is not kept out of reach of children, children may pull the lift cord for fun. In case the lift cord is hung on a child""s head, a fetal accident may occur.
U.S. Pat. No. 5,103,888 discloses a motor-driven blind, which keeps the lift cord from sight. According to this design, a motor is mounted in the headrail or bottom rail, and controlled by a remote controller to roll up or let off the lift cord. The motor is used to control lifting of the lift cord only. When adjusting the tilting angle of the slats, the user must approach the blind and touch-control a tilting control unit. This operation manner is still not convenient.
The present invention has been accomplished to provide a screw transmission mechanism for a motor-driven blind, which eliminates the aforesaid drawbacks. It is the main object of the present invention to provide a screw transmission mechanism for a motor-driven blind, which controls lifting/lowering of the slats and bottom rail of the Venetian blind as well as tilting of the slats. It is another object of the present invention to provide a screw transmission mechanism for a motor-driven blind, which is compact, and requires less installation space. It is still another object of the present invention to provide a screw transmission mechanism for a motor-driven blind, which is inexpensive to manufacture. To achieve these objects of the present invention, the screw transmission mechanism is installed in a motor-driven Venetian blind and adapted to lift/lower the slats and bottom rail of the Venetian blind and to tilt the slats, comprising at least one cord roll-up unit and a driving unit adapted to drive the at least one cord roll-up unit. Each cord roll-up unit comprises: an amplitude modulation set, the amplitude modulation set comprising a support, an amplitude modulation lift cord connected to the slats and bottom rail of the Venetian blind and adapted to lift/lower the slats and bottom rail of the Venetian blind, and an amplitude modulation wheel threaded into the support and coupled to the driving unit for free rotation and axial movement relative to the support to roll up/let off the amplitude modulation lift cord upon operation of the driving unit, the amplitude modulation wheel comprising a longitudinal groove; a frequency modulation set, the frequency modulation set comprising a frequency modulation lift cord adapted to tilt the slats of the Venetian blind, and a frequency modulation wheel sleeved onto the amplitude modulation wheel and adapted to roll up/let off the frequency modulation lift cord, the frequency modulation wheel comprising a notch; and a linkage, the linkage comprising a link mounted in the groove of the amplitude modulation wheel and the notch of the frequency modulation wheel to couple the frequency modulation wheel to the amplitude modulation wheel for synchronous rotation, and a stop block adapted to limit the angle of rotation of the frequency modulation wheel and to force the link away from the frequency modulation wheel when the amplitude modulation wheel rotated to a predetermined position.